Is he really person #4254 to ask that question?! 🙂
It really is quite a simple problem. If you've built a system that requires auth/login then adding a few extra lines of code and a field in you db shouldn't be that hard should it?
Think about it now. If they've logged in, you must check on your otther pages that the person viewing them is actually logged in. If you are not then there is no purpose to having a login. If you are then you just need to set a db entry to say when someone is logged in and count those entries that are set to logged in. Simple.